The present invention relates to a method for protecting a zone in a power system, which zone comprises a number of transmission lines connected to power sources and a number of transmission lines connected to a number of loads where the power sources and the loads are arranged outside the zone, wherein the method comprises the steps of: continuously measuring all the incoming currents (Iin) to the zone, continuously measuring all the outgoing currents (Iout) from the zone, and continuously calculating the differential current (Id) according to Id=Iinxe2x88x92Iout.
During a number of years there has been a rapid development of power systems and the capacity requirements of these in turn require highly reliable relaying principles for protecting the system or components of the system in case of faults. These protection requirements apply to many parts of the power system such as for example transformer differential protection, motor differential protection, generator differential protection and busbar protection.
In this kind of protection system, the incoming and outgoing currents of a certain protection zone has been measured since these may be used to detect if a fault occurs within or outside the protection zone. In order to measure these currents, so called current transformers, or CT, are used, one on each incoming and outgoing line. Further each line is provided with a circuit breaker for breaking the line in case of a fault. Traditionally the secondary currents of all the CTs are lead to a central differential relay which calculates the differential and the restraint current, compares them and makes the decision whether to trip all breakers of the in- and outgoing lines of the protection zone. In the case of an internal fault, ie a fault within the protection zone, the differential relay should trip the breakers.
If however the differential relay trips all circuit breakers during an external fault, or misoperates in normal operating conditions, this will cause serious technical and economical consequences for the power system.
One solution to this is to have a distributed, or decentralised, algorithm processing principle. This principle is presented in the paper xe2x80x9cImplementation of a distributed digital bus protection systemxe2x80x9d, by He Jaili et al., IEEE Transaction on Power Deliver, Vol. 12, No. 4, October 1997. Here the whole bus protection system is divided into a number of protection units, each installed on one circuit of the bus, ie incoming and outgoing transmission line or transformer. All the protection units are connected by a data communication network. Each unit samples and compares instantaneous values of the differential and restraint currents, and makes a decision whether or not to trip its own circuit breaker.
The low impedance protection algorithm widely used in digital protection systems may be expressed as follows. If for example we suppose a busbar which connects N lines, the differential current Id and restraint current Ir among these lines are expressed as                               I          d                =                  "LeftBracketingBar"                                    ∑                              i                =                1                            N                        ⁢                          I              i                                "RightBracketingBar"                                    (        1        )                                          I          r                =                              ∑                          i              =              1                        N                    ⁢                      "LeftBracketingBar"                          I              i                        "RightBracketingBar"                                              (        2        )                                                      I            d                    -                      k            ⁢                          xe2x80x83                        ⁢                          I              r                                       greater than         D                            (        3        )            
In case of an internal fault, then Id=Ir and equation (3) can be confirmed if proper values to k (k less than 1) and D are chosen. Equation (3) is also known as the percentage differential protection since it introduces the restraint current in order to make the protection more stable for external faults.
In the case of normal loads or external faults, Id should be zero in order for equation (3) to be satisfied and no trip signal will be issued.
However, for external faults, Id will be greater than zero during the saturation period of the CT, causing a misoperation during this time period. Saturation occurs as a result of unpredictably high fault currents, during which, the CT saturates and produces erroneous non-proportional values for the actual current. The main technical problem for the differential protection algorithm is thus misoperation due to external faults principally because the saturation of the CT in the faulted line will produce a picture similar to an internal fault in the measuring circuits, that is to say, the differential current Id will be the same as the restraint current Ir during the saturation period of the CT when an external fault occurs.
Another problem with the above system is that it, due to the above mentioned transformer saturation problems, requires a stabilised restraint current signal to keep stability in the case of an external fault. As a consequence the tripping time is increased to above 20 milliseconds. For many systems it is however required to have faster tripping signals, preferably below 15 milliseconds due to system stability and safety requirements.
One object of the present invention is to provide a fast tripping algorithm for power system protection that remedies the above mentioned problems encountered with present technology.
An additional object of the present invention is to provide a means to guarantee supply of electric power through a protected zone in a case when a fault is external to the protected zone.
The present invention displays a number of advantages over the state of the art. Since the integrated values of the incoming, outgoing and differential currents are used for determining where a fault has occurred, much more stable values are obtained. This means that the evaluation will be more reliable and accurate and that external faults will not wrongly trip the protection device. Because the rate of change values, based on the incoming, outgoing and differential currents, display very characteristic behaviour depending on whether the fault is inside or outside the protection zone, the risk of wrongly tripping the system is substantially reduced.
Further the protection device will not be influenced by current transformer saturation with the present invention. With the algorithm presented, a very fast tripping signal may be obtained that operates either well below the fault current levels or well below the operational time of conventional protection devices.
Another advantage of an embodiment of the present invention is that the method may be used to guarantee power transmission through a protected area in the case of a fault that is external to the protected area. Such a guarantee of assured supply with greatly reduced risk of power outages, brownouts etc, due to faulty tripping in a protected area is of great economic benefit, especially to a supplier of electrical power. Such a guarantee facilitates the avoidance of unnecessary loss of supply leading to extremely expensive consequences in terms of lost production, scrapped production, downtime of expensive plant and so on.
These and other aspects of, and advantages with, the present invention will become apparent from the detailed description and from the accompanying drawings.